To add graphic items to your schedule switch to the "Graphic" tab in the library and drop a graphic file on a layer in the timeline. If desired change the attributes or the duration of the graphic element by using the mouse (in "Edit Mode" only) or in the inspector.

You can also drag and drop files from the Finder. In case just:out is running on another machine, the graphic files must be copied to the other machine using exactly the same path.
It is not possible to add graphic items to the rundown whose drop point is in the past. This would change the current playout and will therefore be rejected. A warning will be shown instead.